import pygame.image
from pygame.sprite import Sprite

class Alien(Sprite):
    """表示单个外星人的类"""

    def __init__(self, ai_game):
        """初始化外星人并设置起始位置"""
        super().__init__()
        self.screen = ai_game.screen

        self.image = pygame.image.load('images/alien.bmp')
        self.rect = self.image.get_rect()

        # 初始位置在左上角
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height

        # 存储水平精确位置
        self.x = float(self.rect.x)

        self.settings = ai_game.settings

    def check_edges(self):
        """如果外星人位于屏幕边缘，就返回True。"""
        screen_rect = self.screen.get_rect()
        if self.rect.right >= screen_rect.right or self.rect.left <= 0:
            return True

    def check_bottom(self):
        screen_rect = self.screen.get_rect()
        if self.rect.bottom >= screen_rect.bottom:
            return True

    def update(self):
        """向左或向右移动外星人。"""
        self.x += self.settings.alien_speed * self.settings.fleet_direction
        self.rect.x = self.x
